Aluminum metal is conventionally produced by the electrolytic reduction of alumina dissolved in a molten cryolite bath according to the Hall-Heroult process. This process for reducing alumina is carried out in a thermally insulated cell or "pot" which contains the alumina-cryolite bath.
As the bath is traversed by electric current, alumina is reduced to aluminum at the cathode and carbon is oxidized to its dioxide at the anode. The aluminum thus produced is tapped off periodically after it has accumulated.
The electrolyte or bath is composed of cryolite (Na.sub.3 AlF.sub.6) containing 1 to 8% alumina. Small amounts of aluminum fluoride, calcium fluoride (4 to 7%) (and sodium carbonate) are added from time to time to maintain the correct bath composition.
Other materials, such as LiF (0 to 7%) have also been added to electrolytic baths, but such baths are indicated to contain only up to 7% excess AlF.sub.3.
Generally, about 7.5 KwH of electricity are required to make one pound of aluminum in this system. Also, generally the voltage drop across a "pot" or cell is 4.0 to 5.0 volts.
One well-known type of cell is known as a "prebaked" type since the carbon anodes have been baked before being put into the cell. Modern prebake cell potlines operate at from 180 to 300 kiloamperes with current efficiencies above 94% and specific energy consumption below 14 kwh/kg aluminum (6.36 kwh/lbAl).
All modern industrial aluminum reduction plants use essentially the same electrolyte chemistry--high excess aluminum fluoride ranging from 8 to 12% AlF.sub.3 and containing 3 to 6% CaF.sub.2. It has been demonstrated in plant tests and is generally accepted that an electrolyte chemistry using high excess AlF.sub.3 contributes to increased metal productivity, i.e., high current efficiency (&gt;94%).
Operating with a high excess AlF.sub.3 chemistry requires improved process controls for the careful feeding of alumina with point feeders and closer monitoring of the cell stability/instability by means of improved computer systems.
Large modern cells operate efficiently because of (1) improved magnetic anode and cathode conductors designed to reduce undesirable magnetic fields and (2) operating with a high excess AlF.sub.3 bath chemistry.
Plant production results clearly indicate that the high excess AlF.sub.3 contributes to increased metal production (high current efficiency &gt;94%) due to the reduction in the equilibrium dissolution of aluminum, sodium, and other metals into the electrolyte from the liquid cathode; consequently, this results in a reduction in the reaction between dissolved metals and CO.sub.2 anode gas in the electrolyte region.
Other older-designed cells, without the more sophisticated modern alumina control technology systems found in the new large modern cells, are generally limited to operating with only 4 to 9% excess AlF.sub.3 in the electrolyte due to difficulties encountered with alumina sludging at higher excess AlF.sub.3 content, and reduced anode-cathode distance due to the high current density design. The metal productivity in these older cells is normally considerably lower i.e. 88 to 93% current efficiency.
Some disadvantages of cell operation with a high excess AlF.sub.3 electrolyte composition include
(1) Higher and more variability in the freezing point, and the corresponding operating temperatures, as the excess AlF.sub.3 concentration can change rapidly in the electrolyte as a result of anode effects, etc. PA1 (2) Increase in the vapor pressure and corresponding fluoride emissions from cells, and PA1 (3) Reduction in the electrical conductivity of industrial baths and increased bath voltage drop, i.e., higher cell voltage, due to increased AlF.sub.3 content.
Modern prebake cells operate with larger anodes to reduce the anode overvoltage and reduce the anode current density to offset the higher voltage drop due to the lower bath conductivity associated with high excess AlF.sub.3 electrolyte chemistry, but problems still remain.
The general operating parameters of alumina reduction cells, and the general chemistry associated with molten cryolite baths are old and well-known and no discussion thereof is needed.